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Lake’s silent killer to be disarmed

By Nicola Jones

After years of waiting, Cameroon is finally able to begin disarming the silent killer that lurks in one of its deadly lakes.

The deep waters of Lake Monoun contain a large amount of dissolved carbon dioxide that could erupt at any time and suffocate people nearby. But researchers have now started installing three pipes that will carefully pump the gas-rich water to the surface.

There, the gas is released from the water little by little and blows away. Pumping is expected to get underway on 14 February and should make the lake safe within about 18 months.

The carbon dioxide comes from volcanic sources underground and dissolves in the waters of Lake Monoun and nearby Lake Nyos. Both lakes are unusually deep and still, trapping the gas-rich water in their depths.

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But if the gas burden becomes too large, or if a disturbance like a landslide stirs the water, the carbon dioxide can be released with explosive force. Both lakes erupted in separate incidents in the 1980s, killing 1700 people at Lake Nyos and 40 at Lake Monoun.

90 per cent saturated

Although Monoun is smaller than Nyos, it is closer to more populated areas and has a higher concentration of carbon dioxide in its waters. At present Monoun is about 90 per cent saturated with the gas. “It has become an urgent problem,” says Joseph Hell, director of Cameroon’s Institute for Geological and Mining Research.

Monoun is also less stable than Nyos, as the bottom waters are only slightly more dense than the water above, making an overturn more likely. “The situation is clearly more dangerous,” says team leader Michel Halbwachs from the University of Savoie in Chambéry, France.

Halbwachs and his colleagues installed a single pipe in Lake Nyos in January 2001 – enough to ensure that gas levels in the lake do not increase, but not enough to make it completely safe. They planned to install a pipe in Lake Monoun at the same time, but did not receive the funds to do so until late in 2002.

Tricky proposition

Monoun’s instability will make degassing it a trickier proposition than at Lake Nyos. The researchers plan to degas the middle layer of the lake first, at a depth of 70 metres, in order to increase the density difference between the middle and bottom waters and make an overturn less likely. The gas will then be sucked out of deeper waters in two stages over the next year and a half.

The operation might prove dangerous for wildlife too. A shallow layer of carbon dioxide gas spewed by the pipes might settle on the surface of the lake, potentially suffocating hippopotamuses and crocodiles that poke their nostrils just above the water to breathe. The team will watch closely for these effects.

A final problem may be the acidity of the water. The pipe in Lake Nyos has been stopped because, the team discovered this week, electronic safety circuitry had corroded more quickly than expected. “We will repair it, but this is a bigger problem than we thought,” says Halbwachs.

The team will need more money to make both lakes safe. Lake Nyos requires four more pipes, says Halbwachs, and all the pipes will need ongoing maintenance.